1. Field of the Invention
Embodiments of the present disclosure relate generally to power conversion, and, in particular, to controlling power conversion in a resonant converter.
2. Description of the Related Art
Resonant power converters consist of a bridge (full or half), a resonant L-C network (i.e., tank), and a rectification circuit. The bridge excites a current in the resonant tank which is rectified into a direct current (DC) output. The rectification acts as a resistive-like load which changes the quality factor (Q) of the resonant tank. By changing the frequency of the bridge voltage, the impedance of the tank network seen by the bridge changes, thus varying the tank current and output power. Tank networks with high Q or networks with a “load independent point” will have large power swings over a relatively narrow frequency range, which can make it difficult for a controller to maintain stability.
Series resonant converters are designed to operate above resonance. At frequencies above the resonant frequency, the tank impedance looks inductive to the bridge and causes a tank current to lag behind the bridge voltage. When a bridge device turns off, this lagging current can be used to charge/discharge parasitic device capacitances to create zero-voltage switching. This is a well known technique for decreasing switching losses and allowing efficient operation of the converter at higher switching frequencies.
In traditional duty cycle controlled converters, one form of cycle-by-cycle control is accomplished through the control of peak current. Because the current waveform in traditional converters is always in phase with the switches, peak current control is a stable method of modulating power. Such a method is not suitable for resonant converters because the phase of the current with respect to the switch turn-off is unknown.
Therefore, there is a need in the art for a method and apparatus for cycle-by-cycle control of a resonant power converter.